This application relates to refrigeration systems and in particular relates to refrigeration expansion control for vapor compression refrigeration cycles.
Household refrigerators typically operate on a simple vapor compression cycle. Such a cycle typically includes a compressor, a condenser, an expansion device, and an evaporator connected in series and charged with a refrigerant. The evaporator is a specific type of heat exchanger which transfers heat from air passing over the evaporator to refrigerant flowing through the evaporator, thereby causing the refrigerant to vaporize. The cooled air is then used to refrigerate one or more freezer or fresh food compartments.
In conventional single-evaporator refrigerators, since the freezer compartment and the fresh food compartment are simultaneously cooled with one evaporator, the temperature of the evaporator must be maintained at a temperature lower than about -18.degree. C., which is typically the temperature of the freezer compartment. Accordingly, an evaporator with a lower temperature than is necessary is used to cool the fresh food compartment, causing the efficiency of the overall system to be relatively low.
In an attempt to address this problem, some conventional refrigerators are designed so that the cool air of the freezer compartment and that of fresh food compartment are completely separated, and an evaporator is provided in both the freezer compartment and the fresh food compartment. Since an additional evaporator is needed, however, the manufacturing cost of the refrigerator is increased and the capacity of the refrigerator is reduced.
Other conventional refrigerators require at least two capillary tubes to control expansion. Each capillary tube is preceded in the refrigerant flow path by an electrically activated valve in order to control liquid discharge from the condenser to selectively flow through one of the capillary tubes. An air flow direction control scheme directs air flow to and from the sole evaporator to be either fresh food or freezer compartment air. When the air flow coupling is with the fresh food compartment (about 5.degree. C.) the refrigerant operates at a relatively high evaporator saturation temperature and when the air flow coupling is with the freezer compartment (about -15.degree. C.), the refrigerant operates at a relatively lower saturation temperature.
Higher evaporator refrigerant saturation temperature is desirable since the higher the saturation temperature, the greater the obtainable cycle efficiency. The cycle efficiency, however, will only be greater for higher temperatures if the evaporator exit state is such that the refrigerant is substantially a saturated vapor. Accordingly, this is the purpose for two switchable capillary tubes. A higher evaporator saturation temperature produces a high pressure, thus a higher vapor density, thereby generating a greater compressor mass flow rate. To support a higher compressor mass flow rate, a less restrictive capillary tube is required. This system will work satisfactorily near operating conditions for which the capillary tubes were optimized. At off design conditions, the evaporator exit state will be either a vapor quality or superheat and cycle efficiency will be lower.
Therefore, it is apparent from the above that there exists a need in the art for a simplified refrigeration expansion control. It is the purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.